1. Field of the Invention
This invention relates to fiber optic systems and methods for detection and location of acoustic, ultrasonic, mechanical, and other disturbances. In particular, it relates to a system for detecting, locating, and identifying an intrusion upon a distributed fiber optic sensor.
2. Description of the Related Art
Distributed optical fiber sensing systems based on the Sagnac interferometer and the Mach-Zehnder interferometers have been developed. These systems depend on the interferometric detection of phase differences between two optical signals whose relative phases have been shifted by changes in the optical properties of their respective paths caused by the acoustical, mechanical, or other measure and of interest. The change in optical property of the fiber path may be in the form of elongation, change in index of refraction, change in birefringence, or a combination of these or related effects.
With modern low loss fibers and solid state laser diode sources, it has become possible to develop systems having single sensing loops up to several tens of kilometers in length. In one field of application, these fiber sensing loops can, for example, be placed under carpets, imbedded in walls, under roads, or under turf. In such installations the sensors can be effective to detect the intrusion of personnel, vehicles, animals, etc. into a protected area of interest. Another application of interest is to use one or more of the fibers within a telecommunications or power cable as the distributed sensor element to provide information as to contact with or intrusion into the cable anywhere along its length. Such an arrangement would be most valuable for protection of the cable facilities of power or telephone companies against unauthorized intrusion or tampering. When deployed in this manner, the sensor can detect direct contact, mechanical pressure, or acoustic signals. By virtue of the interferometric phase sensing, such sensors have been found to have very high useable sensitivity over great lengths.
While these distributed sensor systems are valuable in detecting events over large areas, they have several limitations. In particular, there are no convenient and economical means for determining the location of an event along the extensive sensing loop. A simple approach to this problem has been the breaking of a larger perimeter protecting loop into a number of smaller loops, each protecting a more limited zone. This however defeats a major cost advantage of long sensors in that separate sources, couplers, detectors, and connections are required for each zone. Moreover, such segmenting is not convenient or economic in long distributed systems such as intercity telephone or power cables. Other means have been devised to overcome this limitation by complex means of signal processing as described by Eric Udd in U.S. Pat. No. 4,976,507, "Sagnac Distributed Sensor". There he describes a systems wherein the severity and location of a time varying disturbance are determined along a fiber by operating a Sagnac interferometer either simultaneously or sequentially in two distinct modes. In the first mode the interferometer is operated where the counterpropagating light signals in the Sagnac loop operate at the same wavelength such that when an event impinges on the fiber, the relative phase shift generated between the counterpropagating light beams will depend on the location of the event around the loop and the severity of the event. In the second mode, an optical frequency shifting element is activated in one side of the sensing loop near the source. The first light beam which propagates from the source through the short side of the loop near the frequency shifter is shifted up in frequency by a slight amount before it passes out over the active sensing portion of the loop. The other counterpropagating beam passes through the distributed sensing loop first, and is then shifted up in frequency by the same amount just before passing back into the common source. This has the effect that the two beams are at slightly different frequencies at any point out on the active sensing loop, but are at the same higher frequency when they recombine at the detector. According to the disclosure, in this mode a disturbance to the fiber will cause a relative phase shift to the two beams of different frequency and the magnitude of the phase shift is determined by the severity of the disturbance. However, the magnitude of the phase shift is not affected by the position of the disturbance along the loop. Through complex processing of the information from the two modes, the information as to location and severity of disturbance may be separated. This method has the disadvantage of requiring an expensive discrete optical frequency shifter with associated control circuitry, and complicated signal processing circuitry. In cases where only notice of an event and its location is desired without severity information, a simpler and less expensive means for location is needed.
It is often desirable in intrusion detection security systems to know when authorized persons have entered an area for legitimate purposes and to record their identity. In systems providing surveillance of spaces on campuses or within buildings, it is regularly necessary for authorized persons to enter monitored areas without setting off reactions to alarms. Also, telecommunication or power cables which may be protected by such a system must often be accessed by authorized maintenance personnel of the operating company. In such cases it is desirable to properly identify and ignore the authorized person remotely without having to utilize another communication system, or shut the entire protective system down during access period. The prior art has not yet provided means to provide this type of identification. An economic method which utilizes the intrinsic capabilities of the distributed interferometer sensor is needed and has not been provided by the prior art.
An important requirement of intrusion detection systems in security and facility protection applications is the ability to regularly and conveniently proof test the integrity of the sensing loop by remote means. In a distributed Sagnac sensor, a break in the sensing fiber is of major concern. If the fiber were broken while in service, a momentary event would be recorded, and then the system would quickly return to a quiescent baseline state. Since no signal could propagate in either direction around the loop, there would be no signals other than background noise returning to the detector, and the output would look like that from a normal undisturbed system. What is required is means to remotely perturb the system in a way which positively proves that the sensing loop is intact and operating normally. Prior systems have not provided such a capability.
While interferometer based sensor systems have been developed with a number of refinements, prior systems have not been optimized for use as intrusion detection systems in security applications. What is desired and not yet available is a fiber optic sensing system which provides economic means for location of events, recognition and identification of authorized intrusions, and positive means for periodically and remotely proof testing the integrity of the sensing fiber loop.